Waveguide mode-suppressing filter



May 5, 1964 L. LEWlN 3,132,314

' WAVEGUIDE MODE-SUPPRESSING FILTER Filed Feb. 25, 1960 r- 4 a I l /l 2 nventor Z. LEW//V United States Parent Ofi 3,132,3l4 P atented May 5, 19 64 ice 3,132,314 WAVEGUIDE MODE-SUPPRESSING FILTER Leonard Lewin, London, England, assignor to International Standard Electric Corporation, New York, N.Y. Filed Feb. 23, 1960, Ser. No. 10,318 Claims priority, application Great Britain Mar. 3, 1959 8 Clai'ns. (Cl. 333-98) This invention relates to electrical high frequency waveguide systems of the kind in which energy is transmitted through a tubular guide in a 'given wave mode having a magnetic field Component parallel to the longitudinal axis of the guide (H mode). More particularly, the invention relates to systems of the said kind in which tor the sake of high trans-mission efliciency use is made of guides which are overmoded i.e. of such large cross-section as to be capable of supporting not only said .given mode, but also spurious modes of higher order than said given mode. These spurious modes may be generated in the guide by mode-.conversion of the energy being transmitted through the guide in the desired mode, and their presence increases the effective attenuation of the system.

It is therefore an object of -the present invention to provide a waveguide filter arrangement which will pass with little attenuation, energy propagated in a desired H mode of low order but will substantially attenuate energies propagated in H modes of high order.

To achieve this object there is provided within the waveguide a thin layer of magnetically lossy material extending parallel to the longitudinal axis of the waveguide with its surfaces in a region of the guide in which the intensity of the longitudinal magnetic field arising from propagation in the desired =1ow order H modes is substantially at its minimum value. With this positioning the magnetically lossy layer introduces only negligible attenuation of the low order mode energy. This same positioning however is one in which the intensities of the longitudinal magnetic fields arising from the high order H modes are well removed cfrom their minimum values, and the high-order-mode energies are therefore subject to attenuation by absorption in the magnetically lossy layer.

The principles of the invention will be bet-ter understood from the following description of a particular embodiment, taken in conjunction with the accompanying drawing the single figure of which illustrates diagrammatically in longitudinal section a mode-suppressing filter for use in a long-hani waveguide communication system of the kind in which the communication energy is normally transmitted in the dominant circular H mode through an over-moded tubular guide of circular crosssection. In the drawing only such elements are shown as are essen-tial to an Understanding of the invention.

Referring now to the drawing, there is indicated in section at 1 a length of said overmoded circular waveguide, of interior radius R. Coaxially `disposed within the 'guide 1 is a thin cyclinder 2 of material having low dielectric but high magnetic loss, this cylinder being made trorn a solid piece of suitable nickel-Zinc ferrite, and having radius r=0.62 8*R, and a length L o f the order of twenty wavelengths. M-agnetically lossy cylinder 2 is firmly held between the inner surface of -a coaxi-al cylinder shell 3 which is itself a tight fit within guide 1, and the surface of a coaxial inner solid cylinder 4. Both sheet '3 and solid cylinder 4 `are made of material having low dielectric and magnetic loss, such as expanded polystyrene.

By making the radius r equal to 0.628 times the waveguide radius R the position of the magnetically lossy cylinder 2 is one in which the longitudinal magnetic field due to the H mode energy is of substantially zero intensity, and there is therefore little absorption of H mode energy by the cylinder '2. In this same region, however, the longitudinal magnetic field due to any Hog mode energy in the guide :1 is close to its maximum intensity, and the fields due .to energies in the H mode and in modes of still higher order all have non-zero intensities, with the result that the lossy cylinder 2, absorbs quite appreciable amounts of energy from the Hoz and higher order modes. The device thus functions as a pass filter 'for H mode energy and as an absorption filter tor energy in the H z mode and higher order modes.

The two ends of the magnetically lossy cylinder 2 may be circumferentially serrated in saw-tooth fashion so as to taper any impedance rmismatch between the filter and the remander of the guide. For a Wavelength of 8 mm. the teeth of the serrations would be about three wavelengths long, each tooth tapering from a sharp point to,

a root dimension of the order of one half to one whole wavelength.

In the foregoing description the magnetically lossy cylinder 2 has been described as made from a solid piece of nickel-Zinc ferrite. I-t is to be understood however that any constr uction which provides a layer having low `dielectric loss combined with high magnetic loss may be used. In the present embodiment such a layer may be formed, for example, by winding magnetic recording tape round'the solid cylinder 4, or even by painting either cylinder 4 or the inner surface of shell 3 with a slurry containing magnetic material such as powdered ferrite.

In some cases it may be useful to bias the magnetic condition of the lossy layer by a controlling steady field, and this may be provided either by external means or by using magnetic material in `'a magnet-ised condition.

If it is known or suspected tha-t energy is being converted from the normal mode to a high order mode at some particular point in a waveguide system, a modesuppressing filter should be inserted, not just at that point, but further along the 'guide in the direction of forward propagation by a distance of the order of say wavelengths. -In a long run of waveguide it may be desirable to insert mode-suppressing filters with a wide spacing interval even though specific points of mode con- Version are not suspected.

While the principles of the invention have been described above in connection with one specific embodiment, it is to be clearly understood that this description is :made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. A high frequency waveguide mode suppressing filter arrangement for favoring the TE mode over all other waveguide modes, comprising a circular waveguide having an internal radius R, said radius being suflicient to support the passage of wave energy in both the desired low order circular H mode and at least one undesirable higher order H mode, said filter comprising a multi- Wavelength cylinder of magnetically lossy material disposed co-axially within said guide, means whereby the magnetic cylinder is provided With a radius such that a traveling TE wave contain substantially no longitudinal Component of the magnetic vector, said cylinder having a radius r, smaller than R, such that it occupies a region in said circular waveguide where the longitudinal magnetic field of the desired low order H mode is a minimum, and means for supporting said cylinder in said circular waveguide.

2. A filter arrangement according to claim 1 where the mean radius r of said cylinder is substantially equal to 0.63R.

3. A filter arrangement according to claim 1 wherein the length of said cylinder of magnetically lossy material is of the order of twenty wavelengths.

4. A filter arrangement according to claim 1, wherein said magnetically lossy cylinder is firmly held between the inner surface of a cylindrical shell which is itself a tight fit within said waveguide and the surface of a coaxial solid cylnder which is itself a tight 'fit within said magnetically lossy cylinder, said cylindrical shell and said solid cylinder both being formed from a material having low dielectric and magnetic lose.

5. A filter arrangement according to claim 4, whe'ein said cylindrical shell and said solid cylnder are formed in expanded polystyrene.

6. A filter arrangement according to claim 1 wherein said magnetically lossy material has low dielectric loss.

7. A filter arrangement according to claim 6, Wherein said magnetically lossy material is a nickel-Zinc ferrite material.

8. The wavegude filter` of claim l Wherein the loss of said lossy material occurs as a result of domain efiects. 20

References Cted in the file of this patent UNITED STATES PATENTS 2,270,949 Hulster Jan. 27, 1942 2,760,l71 King Aug. 21, 1956 2,762,98l Morgan Sept. ll, 1956 2,764,743 Robertson Sept. 25, 1956 2,948,870 Clogston Aug. 9, 1960 2,951,220 Miller Aug. 30, 1960 3,041,559 Kompfner June 26, 1962 FOREIGN PATENTS 603,1l9 Great Britain June 9, 1948 OTHER REFERENCES October 1956, page 

1. A HIGH FREQUENCY WAVEGUIDE MODE SUPPRESSING FILTER ARRANGEMENT FOR FAVORING THE TE01 MODE OVER ALL OTHER WAVEGUIDE MODES, COMPRISING A CIRCULAR WAVEGUIDE HAVING AN INTERNAL RADIUS R, SAID RADIUS BEING SUFFICIENT TO SUPPORT THE PASSAGE OF WAVE ENERGY IN BOTH THE DESIRED LOW ORDER CIRCULAR H MODE AND AT LEAST ONE UNDESIRABLE HIGHER ORDER H MODE, SAID FILTER COMPRISING A MULTIWAVELENGTH CYLINDER OF MAGNETICALLY LOSSY MATERIAL DISPOSED CO-AXIALLY WITHIN SAID GUIDE, MEANS WHEREBY THE MAGNETIC CYLINDER IS PROVIDED WITH A RADIUS SUCH THAT A TRAVELING TE01 WAVE CONTAIN SUBSTANTIALLY NO LONGITUDINAL COMPONENT OF THE MAGNETIC VECTOR, SAID CYLINDER HAVING A RADIUS R, SMALLER THAN R, SUCH THAT IT OCCUPIES A REGION IN SAID CIRCULAR WAVEGUIDE WHERE THE LONGITUDINAL MAGNETIC FIELD OF THE DESIRED LOW ORDER H MODE IS A MINIMUM, AND MEANS FOR SUPPORTING SAID CYLINDER IN SAID CIRCULAR WAVEGUIDE. 